Electrodeposition of lead and polytetrafluoroethylene



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Y ELECTRODEPOSITION'OF LEAD AND POLYTETRAFLUOROETHYLENE I Filed DBC. 4' 1964 .im 11 Kiwi Q VIU k United States Patent O U-S. Cl. 204-38 6 Claims ABSTRACT OF THE DISCLOSURE The invention provides a method of electrolytically coating a metallic substrate with lead and polytetrafluoroethylene in which the `substrate is the cathode in an electrolytic bath which contains a lead salt in solution together with polytetratluoroethylene particles in suspension, the cathode being connected to the negative side of a D.C. electric source, the bath having an anode connected to the positive side of said source.

This invention -comprises improvements in or relating to coating substrates with compositions of matter containing two or more components.

A particular use of such coated substrates is as a bearing material, the compositions of matter in this case having anti-friction properties.

It has been shown that a desirable bearing material may consist of two, or more, metal or suitable nonmetallic components associated as a layer possessing antifriction properties and in some cases supported on a relatively strong backing material. Conventional methods of casting, powder metallurgy and electrolytic deposition have all been used in the manufacture of such bearing materials. However certain metals, which may be required in association to make a bearing material, may in fact prove incompatible when attempts are made to apply them together by such conventional methods on to the backing material.

According to the present invention, a substrate is coated with a composition of matter comprising as components a metal and a polymerised resinous compound having a low coefficient of friction suitable for bearings, the latter component at least being applied electrophoretically to the substrate.

One example of such a polymerised resinous compound which is very suitable for use in bearings is polytetrailuoroethylene.

The electrophoretically deposited component may be 4 applied to the substrate simultaneously with `another and electrolytically deposited component. For instance lead may be deposited electrolytically whilst polytetrafluoroethylene is being deposited electrophoretically. Alternatively, a metal and the polymerised resinous compound may be deposited together, both Ibeing deposited electrophoretically; an example of `such deposition is when a substrate is coated with aluminium and polytetrafluoroethylene.

The methods of this invention as above set forth may for example be usefully employed in the manufacture of specialized bushing materials, for example those which do not require a liquid lubricant but have an indented backing whereof the indentations are filled with dry lubricant and a Vehicle for the lubricant. In such a use, the backing may lirst be rolled to indent it and the indentations filled, or the indented surface coated, by the methods 3,434,942 Patented Mar. 25, 1969 ICC above set forth. Lead and polytetrauoroethylene com`- positions are suitable for this purpose, but since polytetrauoroethylene is expensive, this composition may be replaced partially by compositions containing graphite or indium.

In one preferred method according to this use of the invention, a lead-coated roll-indented composite strip has its lead coating applied electrolytically and the lead/ polytetrafluoroethylene is applied by simultaneous electrolysis and electrophoresis `onto the roll-indended lead coating.

Alternatively, the electrolytic deposition and electrophoretic deposition may be effected in successive steps, the electrophoretic deposition of one component conveniently being effected on a freshly electrolytically deposited layer of another component.

Some examples of the methods of this invention will now be given by way of illustration.

EXAMPLE I A steel strip is first coated with a cladding layer of bearing alloy of the copper/lead/tin type, its composition being say copper, 10% lead and 10% tin, the proportions being by weight. After suitable sizing as by machining, this .strip is indented by means of a roll whose prole is fashioned in such a way as to produce rectangular pits in the surface of the copper based cladding layer. Such indentations will in general be relatively large but are not essentially so, as micropitting, e.g. as described in British patent specification No. 522,685, can also be employed if desired.

The indented strip is cleaned conventionally and then plated with lead from any of the well-known electrolytes hitherto employed for electrolytic deposition of lead.

A plated layer of 0.2 to 2.0 x 103 ins. thickness is suitable.

Immediately after washing from the electrolytic deposition stage, the strip is passed between wipers and enters an electrophoresis cell which contains fior example la suspension of PTFE (I.C.I. Fluon G.P. l dispersion). The strength of this dispersion can be reduced if desired when a great speed of electrophoretic deposition is not required; for instance one part of the dispersion is mixed with four parts of distilled or deionised Water. A direct current of some 5-10 ma./sq, cm. is passed between the lead plated indented strip and a counter electrode of steel which is connected to the negative terminal of the D.C. source. Under such conditions at room temperature a rate of deposition, reckoned as dry material, of the order of 0.2 gin/Coulomb is easily achieved.

The PTFE deposit is produced in a semi-dry condition and is subsequently further dried by infra-red radiation and then compacted -by simultaneous rolling and heating to a temperature of around 320 C. The :strip is then quenched. This stage is followed by a skivlng operation to produce a smooth lead/PTFE surface.

The resulting material provides a surface eminently Suitable for use in dry bearings such as are encountered in the steering mechanism of motor-cars and the like.

In cases Where corrosion might be a factor, it is desirable to employ a material less subject to acid attack than' lead. In such cases an alloy of lead with for example tin or indium would be prepared. This can of course be made by electro-deposition or other means. A convenient bath for lead-indium alloy deposition is described lin U.S. patent specification No. 3,067,110.

Where it is required to reduce the amount of PTFE for any reason it may be substituted partially by for example graphite or molybdenum disulphide.

EXAMPLE 2 The material is produced as described above except that the electrophoresis bath is as follows:

I.C.I. Fluon G.P. 1 dispersion 'ml 200 Colloidal graphite paste (Aquadag) grs 10() made up to 1 litre with distilled or deionised water. A current density of -10 ma. per sq. cm. is suitable. A deposition rate of 0.05 ,gm/Coulomb is achieved and the deposited mixture contains 91.4% by weight of polytetrauoroethylene when the process is carried out at room temperature.

EXAMPLE 3 An indented bimetallic strip is prepared as for the electro-deposition of lead, or `a lead alloy, rinsed, passed through Wipers, and fed into a combined electrolysis/ electrophoresis cell which contains a solution as shown below:

Lead (as sulphamate) gms 100 Fluon G.P. 1 dispersion ml 200 Water, distilled or deionised, to make 1 litre. The pH value of the bath is adjusted to be in the range 1.0-4.5 (4.0-4.5 being preferred) by addition of ammonium hydroxide or sulphamic acid. The prepared bimetallic strip is connected to the negative terminal.

The deposition is carried out at 50 C. at a current density of 50 ma. sq. cm. `and at a rate of 0.0012 gm./ Coulomb. The deposit contains about by weight of polytetratluoroethylene. The combined deposit is next dried, heated at 320 C., quenched and rolled and finally skived.

EXAMPLE 4 Higher PTFE contents of the combined deposit may 'be obtained by increasing the Fluon G.P. 1 dispersion content of the deposition bath. A strip as used in Example 3 is fed into a cell containing a solution as shown below:

Lead (as sulphamate) gms 100 Fluon G.P. 1 dispersion mls 300 Water, distilled or deionised, to make 1 litre.

pH value of the bath as prepared was 4.5.

The deposition is carried out at 50 C. at a current density of 50 ma. per sq. cm. The deposit contains about 45% -by weight of PTFE.

In general, however, it is preferred not to exceed about 10% by weight of PTFE in the combined deposit because of the exfoliation of the latter which may take place upon rolling. The exfoliation depends to a large extent on the amount of reduction in thickness of the combined deposit together with the amount of PTFE present.

The PTFE content of a mixed deposit tends to increase with time of deposition, other conditions being held constant.

The initial deposit contains little PTFE and consists almost entirely of lead, thus providing a substantial bond to the substrate as in conventional plating.

As deposition proceeds the PTFE on the cathode tends increasingly to insulate this electrode from the lead ions in solution and hence the process favours increasingly the electrophoretic deposition of the PTFE to the exclusion of the eleetrolytic deposition of lead.

The PTFE content of a deposit can therefore be varied according to the current density, composition of the `bat-l1 and time of deposition. The current density is generally limited according to the practice in the electrolytic deposition of lead, and in practice is the highest possible in the particular circumstances before the onset of dendritic growth and is one therefore that would in the absence of PTFE in the bath give a smooth compact deposit of lead.

4 EXAMPLE 5 Aluminum and PTFE are electrophoretically deposited simultaneously from a liquid medium made up as follows:

Fluon G.P. 1 suspension -ml 200 Al powder-300 mesh (BSS.) gm Make -up to l litre with distilled or deionised water.

This bath may be operated at a current density of 5 ma./sq. cm. with the work as the anode connected to the positive pole of the D.C. Supply. The cathode may conveniently be of steel. A rate of deposition of 0.15 gm./coulomb is attained at room temperature. The cornbined deposit of PTFE and aluminium contains around 16% by Weight of aluminium.

EXAMPLE 6 Another liquid medium for use in the simultaneous deposition contains:

Fluon G.P. 1 dispersion ml-- 100 Aluminum powder- 300 mesh (BSS.) gm 200 Make up to 1 litre with distilled or deionised water.

A current density of 5 ma./sq. cm. yields around 0.2 g./coulomb of deposit containing 37.3% by weight of aluminum.

When higher contents of aluminum are required in the deposit a substantially non-aqueous type of liquid, for example commercially available methylated spirit, may be used instead of water.

The present invention makes use of the phenomenon of elecrtophoretic deposition to produce non-homogeneous bodies.

EXAMPLE 7 An aluminum/PTFE composite 'bearing material may be produced by electrophoretic deposition from a liquid medium of the following composition:

Al powder-300 mesh (B.S.S.) gm 100 Fluon G.P. 1 suspension ml 10 gm..- 2 Water rnl 40 The whole is diluted to 1 litre with commercial methylated spirit. The Work is connected as cathode to the negative pole of the D.C. source and a rate of deposition of about 0.01 gm./coulomb is achieved at room temperature with a cathode current density of 2.5 ma./cm.2. The resulting material contains 94.2% PTFE.

EXAMPLE 8 A steel strip 0.060 inch thick, which has a 0.022 inch thick cast-on layer of bronze whereof the composition is 8% lead, 4% tin, 4% zinc, and the remainder copper, is cleaned with trichlorethylene to remove any grease and, if necessary, is treated, for example by scratch brushing, to remove gross superficial soil, or by milling to remove lead sweat.

The cleaned strip is then rolled with a roller having a knurled surface (a British grade two knurl is suitable) to obtain a closely indented surface with a coined or polished finish due to the metal filling the indentations in the roll.

The strip is degreased in trichlorethylene vapour and is cleaned anodically in an alkaline cleaner of the following composition:

Gms./l. Sodium hydroxide 21 Sodium carbonate 15 Trisodium phosphate 7.5 Sodium metasilicate 15 Ethylenediaminetetra-acetic acid 10 The cleaning is carried out 40-50 with a current density of 25 to 30 amps/sq. ft.

After such cleaning the strip is rinsed.

The strip is now treated in a bath as set forth in Example 4 for the simultaneous electrolytic deposition of lead and electrophoretic deposition of PTFE, the deposition being carried out at room temperature, with current density of 50 ma. per sq. cm. The strip is rinsed and dried.

Due to the form of the indentations produced by knurling, it is found that the exfoliation dicultes discussed in Example 4 are greatly mitigated. For instance, a lead/PTFE composite containing 34% by weight of PTFE was rolled satisfactorily to reduce the thickness of the composite to 0.076 inch.

The method of Example 8 is illustrated diagrammatically in FIGURE 1 and one form of subsequent rolling and heating treatment is illustrated in FIGURE 2.

The above are merely examples of specific non-limitative embodiments of the present invention. Other sequences of deposition than those of Examples 1 to 4 and 8 may be followed by heat-treatment and rolling, or by rolling directly after deposition, or by rolling after a preiminary short heat treatment and following this another more prolonged heat treatment at a higher temperature. Again in some cases, the quality of the material produced by the invention may further be improved by a nal burnishing or rolling operation.

Bushes and bearings of the so-called dry lubricated type manufactured from our PTFE lead/ lead bronze/ steel material have been shown to perform over a wide range of applications and to overcome many of the limitations normally associated with earlier types of dry lubricated bearings.

In certain applications, the coated substrate may be machined to fine limits as for example `by boring or broaching. This machining may be particularly desirable when the coated substrate is used for manufacture of bushes or bearings and in this case the machining may be performed at any desired stage of the bush or bearing manufacture.

I claim:

1. A method of coating a metallic substrate with a mixture of lead and polytetraliuoroethylene having a low coetcient of friction suitable for bearings comprising i11- troducing the substrate into an electrolytic bath containing a solution of a lead sulphamate in which is dispersed a suspension of polytetrauoroethylene particles, and connecting the metallic substrate to the negative side of an effectively direct current electric source and-which bath has an anode connected to the positive side of said electric source.

2. A method according to claim 1, wherein the substrate is indented prior to deposition of the coating.

3. A method according to claim 1, wherein the indentation is effected by a knurling operation.

4. A method according to claim 1, wherein, subsequent to coating, the coated substrate.` is subjected to a rolling operation.

5. A method according to claim 1 wherein the coated substrate is subjected to iinal burnishing.

6. A method according to claim 1 comprising also machining the material to fine limits, for example as by boring or broaching.

References Cited UNITED STATES PATENTS 1,702,927 2/ 1929 Bezzenberger 204-181 2,707,703 5/1955 Dorst 204-181 2,830,017 3/1958 Shyne et al 204-181 2,861,935 11/1958 Fahnoe et al 204-181 2,878,140 3/1959 Barr 204-181 2,994,654 8/ 1961 Fahnoe et al 204-181 2,999,798 9/ 1961 Eitel et al. 204-181 JOHN H. MACK, Primary Examiner.

E. ZAGARELLA, Assistant Examiner.

U.S. Cl. X.R. 204--181 

